The present invention relates to an electrophotographic printing apparatus which obtains a print by fixing a toner image formed on the surface of a print member and a printing system to print on the double-side surfaces of a print member using said electrophotographic printing apparatus.
An electrophotographic printing system of the double-side printing type has been proposed. In such a system, a paper reversing apparatus to reverse a print paper is installed between two electrophotographic printing apparatuses of the one-side printing type arranged side by side. An image is first printed on the front side surface of a print paper using an electrophotographic printing apparatus in a first stage. The print paper having a printed image on the front surface is reversed using said paper reversing apparatus, and the print paper is supplied to an electrophotographic printing apparatus in a second stage to print an image on the back side surface of said print paper. In a printing system of this type, a controller to operate two electrophotographic printing apparatuses in linkage with each other is described in U.S. Pat. No. 4,774,524 (1988). A print paper transporting apparatus and a fixing apparatus disposed between two electrophotographic printing apparatuses are described in U.S. Pat. No. 5,179,417 (1993).
An electrophotographic printing apparatus having a high printing speed provides preheating means in front of a fixing unit. The preheating means preheats a print paper when its preheating plate comes into contact with the surface of the paper opposite to the surface on which a toner image is transferred. In an electrophotographic printing system constructed to print images on both sides of a print paper, there is a fixed toner image (picture) fixed in the printing process of the electrophotographic printing apparatus in the first stage on the surface of the print paper opposite to the surface on which an image is to be printed by the electrophotographic printing apparatus in the second stage. Therefore, the preheating means of the electrophotographic printing apparatus in the second stage preheats the printing paper by contacting the surface on which the fixed toner image is formed. When the paper is excessively preheated, the fixed toner image is softened again and attaches to the preheating plate and the margin of the print paper so as to stain the apparatus and degrade the printing quality.
The amount of preheating by the preheating means in a electrophotographic printing apparatus of the one-side printing type is generally set under an assumption that there is no fixed toner image on the surface of a print paper contacting the pre-heating plate. In order to realize a good fixing characteristic in a high speed printing, it is preferable that the preheating means preheat the print paper up to a temperature at which the toner image is softened. Therefore, the preheating plate is heated up to a fairly high temperature. In a case where an electrophotographic printing system of the double-side printing type is constructed using two electrophotographic printing apparatuses of the one-side printing type having preheating means for preheating a print paper up to such a high temperature, the fixed toner image is softened again and attaches to the preheating plate and the margin of the print paper so as to stain the apparatus and degrade the printing quality, as described above.
A countermeasure for preventing the softening of fixed toner image can be realized by lowering the preheating temperature of the pre-heating means for the print paper. However, it is impossible to construct an electrophotographic printing system of the double-side printing type having high speed printing by using a electrophotographic printing apparatuses having a low preheating temperature.
If an electrophotographic printing system of the double-side printing type is constructed by using an electrophotographic printing apparatus having a high preheating temperature in the head first stage and an electrophotographic printing apparatus having a low preheating temperature in the second stage, it is necessary to provide two kinds of electrophotographic printing apparatuses. In addition to this, the heat added for fixing a toner image in the electrophotographic printing apparatus in the first stage remains in the print paper supplied to the electrophotographic printing apparatus in the second stage. Therefore, the preheating means used in the electrophotographic printing apparatus in the second stage is required to be set to a smaller amount of preheating while taking the retained heat into consideration. When one of the two electrophotographic printing apparatuses in the electrophotographic printing system is independently used for one-side printing, the electrophotographic printing apparatus in the second stage performs an image printing on print paper which is at room temperature, i.e. without having any retained heat. Therefore, in the preheating means set to a low amount of preheating in consideration of the retained heat, the printing quality is degraded by a decrease in the fixing force of the toner image due to the low preheating temperature of the print paper.